The conventional electric machines—motors and generators—consist of a stationary component (the stator) and a rotating component (the rotor). One of these components is arranged to operate as the field and the other—as the armature of the electric machine. When a conventional electric motor is energized and electric current flows through its windings, the interaction between the electromagnetic forces of the field and armature propels the rotor to rotate and generates mechanical energy. When the rotor of a conventional electric generator rotates, driven by external mechanical forces, a magnetic field rotating in relation to the armature windings cuts through the armature windings and produces voltage at the output terminals of the generator.
Dual-rotor electric motors, though seldom used, are well known in the art. Also known as double-rotor motor or stator-less motor, a dual-rotor motor has two rotors and no stator. In a dual-rotor motor, both rotors are supported coaxially in a stationary motor enclosure. Usually, a smaller inner rotor is supported inside a hollow larger outer rotor. Typically, the shaft of one of the rotors extends through one of the side flanges of the motor enclosure in one direction, and the shaft of the other rotor extends through the other side flanges of the motor enclosure in the opposite direction. One of the rotors of the dual-rotor motor is arranged to operate as a motor field and the other rotor is arranged to operate as a motor armature, similarly to the field and armature of a conventional motor of the same electrical kind. When a dual-rotor motor is energized and electric current flows through its windings, the interaction between the electromagnetic forces of the field and armature propels the two rotors to rotate in opposite directions and generates mechanical energy. If a dual-rotor motor is arranged to also selectively operate as an electric generator, when the two rotors of the machine rotate in opposite direction driven by external mechanical forces, a magnetic field rotating in relation to the armature windings cuts through the armature windings and produces voltage at the output terminals of the dual-rotor motor.
It shall be emphasized that the dual-rotor electric motor is the only known machine that converts another kind of energy into mechanical energy, wherein the internal action and reaction forces can be utilized for producing useful mechanical work.
In this specification, the term “traction motor” is used to designate an electric motor appropriate for vehicle propulsion. The dual-rotor traction motor may be a direct-current (DC) or an alternating-current (AC) electrical machine of various types and designs. The control and protection of a dual-rotor electric traction motor, including directional control, torque-speed output control, over-speed protection of the rotors, thermal protection of the motor, selective operation of the motor as a generator, etc., are generally arranged in the same way and by the same means as in a conventional traction motor of the same electrical kind.
A remarkable feature of the dual-rotor motor is that the torques on the shafts of both rotors always have equal magnitudes and opposite directions, because the rotors are propelled to rotate in opposite directions by the same electromagnetic forces (action and reaction). The two rotors, however, may rotate with different absolute rotational speeds (the speeds in relation to a stationary body, such as the motor enclosure), depending on the external mechanical loads upon each of the rotors.
Another remarkable feature of the dual-rotor motor is that the total torque on the shafts of both rotors, i.e., the sum of the torques of both rotors, is two times greater than the torque on the shaft of an otherwise equivalent conventional motor operating under the same electrical parameters at the same power output. At the same time, the sum of the absolute rotational speeds of both rotors of the dual-rotor motor is equal to the speed of the rotor of the equivalent conventional motor. Said differently, at the same power output, the dual-rotor motor generates two times greater total output torque at a much lower speed of each rotor than the output torque and speed of the rotor of an equivalent conventional motor. Here, the term “equivalent conventional motor” is used to designate a conventional motor of the same electrical kind, having the same size and design of the active electrical components (field and armature), the same power output, and the same electric control as those of the dual-rotor motor. If the outer rotor of a dual-rotor motor is somehow mechanically immobilized, then the dual-rotor motor will operate exactly as an “equivalent conventional motor” as far as the torque-speed output of the motor is concerned.
Usually, the direction of rotation of the rotor of a conventional traction motor is reversible, for selectively producing a backward motion of the vehicle. For the same reason and in the same way, the opposite directions of rotation of the two rotors of a dual-rotor traction motor are reversible.
Most of the contemporary conventional traction motors used in electric or hybrid-electric vehicles are arranged to selectively operate as electric braking generators, for converting part of the kinetic energy of the vehicle into electric energy during speed retardation or braking of the vehicle. Thus, the conventional traction motors provide electric braking with recovery of energy. The dual-rotor traction motor may likewise be arranged to selectively operate as an electric braking generator, generally in the same way and by the same means as a conventional traction motor of the same electrical kind.
The above described unique features make the dual-rotor motors very attractive for applications as traction motors in steerable four-wheeled electric or hybrid-electric vehicles.
When a dual-rotor traction motor is incorporated in a two-wheel-drive system, the two rotors drive the two wheels of a drive axle respectively via two mechanically independent drive trains having the same speed reduction ratios. The two drive trains are arranged to provide the same directions of rotations of the two wheels of the drive axle at opposite directions of rotation of both rotors. In such an arrangement, the dual-rotor motor produces substantially equal tractive forces on both driving wheels, while allowing different rotational speeds of the wheels during a turn of the vehicle. Therefore, a two-wheel-drive system with a dual-rotor traction motor does not require an axle differential. Further, because at the same power output, the total torque of the two rotors is two times greater and the rotational speed of each rotor is two times lower than those of an equivalent conventional motor, the speed reduction ratios between each rotor and the respective wheel is two times smaller than the speed reduction ratio of a drive train using an equivalent conventional traction motor. The result of the described above advantages is a simple, compact, and highly efficient two-wheel-drive system, superior to a drive system with an equivalent conventional traction motor.
When a dual-rotor traction motor is incorporated in a four-wheel-drive system, the two rotors drive the wheels of two drive axles respectively via two mechanically independent drive trains, each one including the final drive and differential of the respective drive axle. The two drive trains may have the same or different speed reduction ratios and are arranged to provide the same direction of rotation of the four wheels of the vehicle at the opposite directions of rotations of both rotors. At the same reduction ratios of both drive trains and the same rolling radiuses of all four wheels, the system provides substantially equal tractive forces on the four driving wheels, while allowing different rotational speeds of the wheels during a turn of the vehicle. At different speed reduction ratios of the drive trains, the system provides tractive forces on the front and rear wheels of the vehicle in the same proportion as the proportion between the speed reduction ratios of the respective drive trains, while allowing different rotational speeds of all driving wheels during a turn of the vehicle. In both cases, however, a four-wheel-drive system does not require an inter-axle (center) differential. In addition, because at the same power output, the total torque of the two rotors is two times greater and the rotational speed of each rotor is much lower than those of an equivalent conventional motor, the speed reduction ratios between each rotor and the wheels of the respective axle is much smaller than the speed reduction ratio of a drive train using an equivalent conventional traction motor. The result of the described above advantages is a simple, compact, and highly efficient four-wheel-drive system, superior to a drive system with an equivalent conventional motor.
In a four-wheel-drive system, when the dual-rotor motor operates as an electric braking generator during speed retardation and braking of the vehicle, the proportion between the electric braking forces on the front and rear wheels is the same as the proportion between the speed reduction ratios of the drive trains from the rotors to the front and rear wheels respectively. If this proportion is selected properly, the dual-rotor motor provides a very safe four-wheel electric braking of the vehicle.
Another advantage of the dual-rotor motor is that, in principle, the smaller inner rotor can rotate with a much higher safe speed than the safe rotational speed of the larger outer rotor. If the dual-rotor motor is incorporated into a four-wheel-drive system, the two rotors may drive the wheels of the two drive axles from standstill to a medium speed. Then, the transmission between the outer rotor and the respective axle may be interrupted via a clutch and the outer rotor may be immobilized by a brake. At this point, the dual-rotor motor continues to operate as a conventional motor, and the inner rotor continues to drive the wheels of one of the drive axles from said medium speed to the maximum speed of the vehicle. Thus, through very simple and efficient mechanics, without shifting of any gears, a drive system with a dual-rotor motor may provide a low-speed/four-wheel-drive mode and a high-speed/two-wheel-drive mode (See inventor's U.S. Pat. No. 6,005,358 Radev).
In spite of the remarkable mechanical simplicity and efficiency of the drive systems for electric and hybrid-electric vehicles using dual-rotor motors, they have not achieved wide application, due to difficulties in two major areas in the construction of the dual-rotor traction motor itself. These difficulties stem from the fact that the two major components of a dual-rotor motor—field and armature—are rotating parts.
The first major difficulty relates to the conduction of electric current from static terminals attached to the motor enclosure to the windings of the rotating outer rotor and to the windings of the rotating inner rotor of the dual-rotor motor. In the known dual-rotor motors, the electric current is conducted from stationary terminals to the windings of the outer rotor through contacts of brushes (held in brush holders attached to the motor enclosure) and sliprings (mounted on the shaft of the outer rotor). The brushes are electrically connected to the terminals of the motor and the sliprings are electrically connected to the leads of the windings of the outer rotor. The brushes are pressed to the sliprings by springs. Such an arrangement is similar to the well known in the art brushes-and-slip-rings conduction of the electric current between terminals and the windings of the rotor of the alternating-current generators. This arrangement is relatively simple and reliable.
However, the conduction of electric current from the outer router to the inner rotor is very complicated in the known dual-rotor motors. The electric current is conducted from terminals attached to the rotating outer rotor to the windings of the rotating inner rotor via very complicated and inaccessible arrangements of brushes and sliprings, or brushes and commutators. Such arrangements require complicated brush holders, capable of counteracting the centrifugal forces for maintaining a substantially constant pressure between the brushes and sliprings. In addition, access to the brush-holders and sliprings in such arrangements is very cumbersome and difficult. All that makes the manufacture and maintenance of the known dual-rotor motors difficult and expensive. Therefore, it will be very beneficial if the dual-rotor motor is arranged so that relatively simple, inexpensive, and accessible means conduct the electric current to the windings of the dual-rotor motor.
The second major difficulty relates to the cooling of the dual-rotor traction motor. The electric motors used for propulsion of steerable vehicles are fully enclosed for protection from moisture and dirt. In a conventional traction motor, the heat generated in the core of the stator is transferred to the motor enclosure mainly by direct conduction, while the heat from the rotor is transferred to the motor enclosure mainly by convection via the circulation of the air caused by the rotation of the rotor. The motor enclosure is cooled usually by an external liquid-cooling or air-cooling system. It is well known in the art that the thermal conductivity of the metals is hundreds of times greater than that of the air. Therefore, the direct conduction of the heat from the core of the stator (where most of the heat is usually generated) to the motor enclosure is a major advantage of the conventional traction motor in comparison with the dual-rotor motor, as far as the cooling of the machine is concerned, because in a dual-rotor motor such direct heat transfer through conduction is impossible.
In a dual-rotor traction motor, the heat from both rotors is transmitted to the motor enclosure mainly by convection through the air between the rotors and the motor enclosure, i.e., through a medium with low thermal conductivity. It is well known that many factors influence the rate at which heat is transferred by convection through a fluid medium between two metal surfaces. The most major of these factors, however, are the areas of the two surfaces, the temperature differences, and the velocity and character of the air flow between the two surfaces.
Therefore, it will be very beneficial if the described above disadvantages, related to the cooling of the dual-rotor traction motor, can be substantially reduced or eliminated by simple, inexpensive, and reliable means.